1. Field of the Invention
The present invention relates to an aluminum alloy casting having high laser weldability and high strength and ductility at a joint. The invention also relates to a technology for improving overall strength, fatigue strength, ductility and so forth of an aluminum alloy structural member subjecting only a part thereof to a thermal treatment (laser processing).
2. Description of the Related Art
In the recent years, in transporting equipments, such as automotive vehicles, railway vehicles, ships, and so forth, for demands better fuel economy and higher speed, lighter weight of structures per se and for lighter weight of structural members to be employed in the light-weight structures have been developed. For light-weight structural member, the members of aluminum alloy are becoming to be frequently employed in place of the conventional steel member for lighter weight.
As the aluminum alloy material, there are variety of materials, such as rolled material, extruded material, cast material and so forth depending upon difference of the production processes. Among these variety of aluminum alloy materials, rolled materials and extruded materials have relatively long stand achievement of use in the railway vehicles and ships. The rolled materials are processes into a desired configuration through press, bending and reducing process and so forth. The extruded material is shaped into a desired configuration of a hollow member and so forth corresponding to a configuration of dies. Such processed members are connected by welding, primarily by arc welding so as to be assembled into a structure.
On the other hand, the aluminum alloy casting has not been positively employed in a welded structure except for repair welding. Also, since the aluminum alloy casting has different mechanical performance of the products depending upon the size, thickness and applied portion and so forth, it may have relatively low reliability. This is also one of the reasons why the aluminum alloy casting has small field of application.
U.S. Pat. No. 4,618,163 discloses an automobile chassis made from extruded light metal tubular rods held together by connecting members also made from light metal. The connecting members are die cast or forged members. It has been known a technology for constructing an automotive chassis structure by connecting the extruded frame of the light metal by means of joints also made of the light metal, for achieving the weight reduction of the vehicle. The above-identified U.S. Patent further discloses employment of the die cast of the light metal as a material for the joints. However, the above-mentioned prior art fails to show what light metal may be used for die cast product nor what component would be a base material for the die cast product. Therefore, the above-identified U.S. Patent does not disclose the material which has superior property to the conventionally known light metal die cast. As mentioned above, the conventional aluminum alloy casting has lower welding performance and mechanical performance than the rolled, extruded or forged material and fluctuates in mechanical performance, application of the conventional aluminum alloy casting for the joint of the automotive chassis is not practically possible.
On the other hand, the aluminum alloy casting holds advantages since it is easily to form three dimensional arbitrary configuration, superior to the rolled material and extruded material. Therefore, it has been desired to develop a technology which may make use of the above-mentioned advantage with improvement of weldability and unevenness of the mechanical performance of the aluminum alloy casting.
Namely, concerning the weldability of the aluminum alloy casting, there have been attempted an improvement of the material per se, such as reduction of impurity elements of the casting or reduction of particle size for microstructure, and an improvement of a welding method by effecting improvement on the material of a welding wire to be supplied. Even through such attempts, the weldability and the mechanical strength of the aluminum alloy casting cannot be satisfactory improved for practical use. Therefore, when the conventional aluminum alloy casting is used for the welding joint, the ratio of strength of the welding joint versus the strength of the base material is relatively low, i.e. in the order of 60 to 80%. Therefore, such welding joint is defective since it may cause breakage at the welding metal portion.
On the other hand, an arc welding method encounters disadvantage for discharging of spatter and so forth to degrade appearance of the joint products to lower the value as a commercial products. If treatment for better appearance is to be performed for better appearance, it is inherent to perform post welding process to cause increasing of the process step and whereby to cause rising of the production cost.
Furthermore, due to high heat transfer coefficient and other property of the aluminum alloy, a large heat input is required for welding. The large heat input causes unnecessarily large amount of reinforcement of weld or to cause increased deformation. This is the critical disadvantage.
On the other hand, laser welding method has been frequently used for welding of steel materials utilizing high energy density, and has been put into practical use as high speed, high efficiency and low distortion welding method. In case of application of the laser welding method for aluminum material, it has not been applied in large scale for the reason that the aluminum material tends to reflect a laser beam and to form a defect, such as blow hole and so forth, at the weld metal.
Concerning the mechanical performance of the aluminum alloy casting, the strength and the ductility of the casting is variable depending upon the composition and type of heat treatment. However, in general, greater strength results in lower ductility, and conversely, increasing of ductility results in difficulty of maintaining sufficient strength. Therefore, in the prior art, it is common to determine the composition and the heat treatment condition for achieving a balance therebetween. Therefore, it has been impossible to increase both of the strength and the ductility of the casting.
For instance, as a technology for improving non-uniformity of the mechanical performance, there is a method for increasing the thickness of the structural member for assuring desired performance, such as strength and so forth. However, the increased thickness of the structural member inherently causes increasing of the weight, and more importantly to cause variation of the mechanical performance in the thickness direction.
It is also possible to perform heat treatment for improving the property of the aluminum alloy casting. By heat treatment, the ductility and strength of the aluminum alloy casting can be improved. However, the heat treatment for overall structure frequently causes deformation of the structure. Furthermore, effecting the heat treatment is not advantageous in viewpoint of the cost.
Furthermore, in the recent technology, there has been employed a structure for certainly provide security for the occupant of the vehicle, i.e. automotive vehicle, railway vehicle, ship and so forth, by providing lower strength at a portion of the structure so that deformation may be caused at the low strength portion upon exertion of impact for absorbing the impact, not only by strengthening a portion of the structure. In this case, it becomes necessary to form a portion having high ductility. Namely, when the impact is exerted, the low strength portion should not cause breakage but may cause deformation for successfully absorbing the impact. Therefore, it is essential to provide high ductility for the low strength portion.
However, it is difficult to provide high ductility for the arbitrarily selected position in the aluminum alloy casting. It should be noted that a method for increasing durability for the aluminum alloy cast component, such as a cylinder head and so forth by re-melting the aluminum alloy casting by irradiating a laser beam for necessary portion, has been disclosed in Japanese Unexamined Patent Publication No. 61-193773. However, in order to achieve satisfactory durability, further improvement, such as for alloy composition and so forth, is required.